全文获取类型
收费全文 | 8604篇 |
免费 | 299篇 |
国内免费 | 237篇 |
出版年
2023年 | 119篇 |
2022年 | 85篇 |
2021年 | 151篇 |
2020年 | 170篇 |
2019年 | 201篇 |
2018年 | 219篇 |
2017年 | 143篇 |
2016年 | 166篇 |
2015年 | 173篇 |
2014年 | 238篇 |
2013年 | 589篇 |
2012年 | 167篇 |
2011年 | 269篇 |
2010年 | 243篇 |
2009年 | 296篇 |
2008年 | 301篇 |
2007年 | 342篇 |
2006年 | 314篇 |
2005年 | 251篇 |
2004年 | 257篇 |
2003年 | 247篇 |
2002年 | 223篇 |
2001年 | 190篇 |
2000年 | 171篇 |
1999年 | 164篇 |
1998年 | 157篇 |
1997年 | 165篇 |
1996年 | 166篇 |
1995年 | 173篇 |
1994年 | 185篇 |
1993年 | 144篇 |
1992年 | 148篇 |
1991年 | 148篇 |
1990年 | 138篇 |
1989年 | 104篇 |
1988年 | 126篇 |
1987年 | 115篇 |
1986年 | 90篇 |
1985年 | 194篇 |
1984年 | 225篇 |
1983年 | 170篇 |
1982年 | 252篇 |
1981年 | 160篇 |
1980年 | 157篇 |
1979年 | 130篇 |
1978年 | 74篇 |
1977年 | 55篇 |
1976年 | 59篇 |
1974年 | 29篇 |
1973年 | 31篇 |
排序方式: 共有9140条查询结果,搜索用时 15 毫秒
921.
The functions of the riboflavin synthesis gene homologues ribA, ribBA, ribC, and ribD from Helicobacter pylori strain P1 were confirmed by complementation of defined Escherichia coli mutant strains. The H. pylori ribBA gene, which is similar to bifunctional ribBA genes of Gram-positive bacteria, fully complemented the ribB mutation and partially restored growth in a ribC mutant. However, ribBA did not complement the ribA mutation in E. coli, thus explaining the presence of the additional separate copy of the ribA gene in the H. pylori chromosome. In E. coli exclusively ribA conferred hemolytic activity and gave rise to production of molecules with fluorescence characteristics similar to flavins, as observed earlier. The E. coli hemolysin ClyA was not involved in causing the hemolytic phenotype. No riboflavin synthesis genes on plasmids conferred iron uptake functions to a siderophore-deficient mutant of E. coli. Marker exchange mutagenesis of the genes in H. pylori was not successful indicating that riboflavin synthesis is essential for basic metabolic functions of the gastric pathogen. 相似文献
922.
Freeze tolerance by various amphibians includes cryoprotectant production in the form of glucose. Activation of the catalytic subunit of liver cAMP-dependent protein kinase (PKAc) facilitates activation of glycogenolysis, a critical biochemical process necessary for production of glucose. Here, we purified PKAc from Rana sylvatica liver to determine the extent to which cold temperature, which stimulates cryoprotectant production, affected PKAc activity and function. PKAc was purified to greater than 95% homogeneity, with a final specific activity of 71 nmol phosphate transferred/min/mg protein. The molecular weight of frog liver PKAc was 47.6 +/- 1.1 kDa and K(m) values for the phosphate acceptor kemptide and Mg-ATP were 9.0 +/- 0.1 and 51.8 +/- 1.0 microM at 22 degrees C, respectively. K(m) values for both substrates dropped significantly at 5 degrees C. The enzyme was sensitive to specific inhibitors of mammalian PKAc (PKA(i), H89) but was only moderately inhibited by high salt concentrations. Furthermore, salt inhibition was reduced at low temperature. The effect of temperature on enzyme activity indicated a conformational change in PKAc at 10 +/- 2 degrees C, with calculated activation energies of 51 +/- 4 kJ/mol at temperatures above 10 degrees C and 110 +/- 9 kJ/mol below 10 degrees C. PKAc in wood frog liver plays a crucial role in mediating the freeze-induced glycogenolysis that is responsible for the production of 200-300 mM levels of glucose as a cryoprotectant. Differential effects of low temperature on enzyme function, increased substrate affinity and reduced ion inhibition, appear to be central to this role. 相似文献
923.
The synthesis of DNA in nuclei and organellar nucleoids at the various stages of somatic embryogenesis in carrot (Daucus carota L. cv. Kurodagosun) was analyzed using anti-5-bromo-2′-deoxyuridine (BrdU) immunofluorescence microscopy. The active syntheses
of both nuclear and organellar DNA started in the cells forming the embryo proper 3 d after the initiation of embryogenesis,
but not in cells forming suspensor-like cell aggregates. In the early globular embryo, active DNA syntheses were continuously
observed in the whole embryo proper, except for the progenitor cells of the root apical meristem (RAM) and shoot apical meristem
(SAM). These were recognized as slowly cycling cells with a non-BrdU-labelled nucleus and strongly BrdU-labelled organellar
nucleoids. At the heart- and torpedo-shaped embryo stages, both nuclear and organellar DNA syntheses were inactive in the
presumptive RAM and SAM. Thus, slowing down of organellar DNA synthesis is not coupled with, but is later than, that of nuclear
DNA synthesis in the progenitor cells of the embryonic RAM and SAM. These findings clearly indicate that the timing of DNA
synthesis is similar in the progenitor cells of both the RAM and SAM in the early stages of somatic embryogenesis.
Received: 18 January 2000 / Accepted: 2 March 2000 相似文献
924.
Kinetic regularities of the enzymatic acyl group transfer reactions have been studied using ampicillin synthesis catalyzed by E. coli penicillin acylase as an example. It was shown that ampicillin synthesis proceeds through the formation of an acylenzyme–nucleophile complex capable of undergoing hydrolysis. The relative nucleophile reactivity of 6-aminopenicillanic acid (6-APA) is a complex parameter dependent on the nucleophile concentration. The kinetic analysis showed that the maximum yield of antibiotic being synthesized depended only on the nucleophile reactivity of 6-APA, the ratio between the enzyme reactivities with respect to the target product and acyl donor, and the initial concentrations of reagents. The parameters characterizing the nucleophile reactivity of 6-APA have been determined. The algorithm of modeling the enzymatic synthesis has been elaborated. The proposed algorithm allows the kinetics of the process not only in homogeneous, but also in heterogeneous (aqueous solution–precipitate) systems to be quantitatively predicted and described based on experimental values of parameters of the reaction. It was shown that in heterogeneous aqueous solution–precipitate systems PA-catalyzed ampicillin synthesis proceeds much more efficiently compared to the homogeneous solution. 相似文献
925.
Novozym 435 (50 g l–1) catalyzed the synthesis of n-octyl--d-glucopyranoside lactate by transesterification between n-octyl--d-glucopyranoside (30 g l–1) and ethyl lactate (100 g l–1) in acetone. In 12 h, a molar yield of 87% n-octyl -d-glucopiranoside lactate was obtained at a overall conversion of 90%. 相似文献
926.
Peter L. Pedersen Young Hee Ko Sangjin Hong 《Journal of bioenergetics and biomembranes》2000,32(4):325-332
This introductory article briefly summarizes how our views about the structural features ofATP synthases (F0F1) have evolved over the past 30 years and also reviews some of our currentviews in the year 2000 about the structures of these remarkably unique enzyme complexes.Suffice it to say that as we approach the end of the first year of this new millinium, we canbe conservatively confident that we have a reasonably good grasp of the overall low-resolutionstructural features of ATP synthases. Electron microscopy techniques, combined with the toolsof biochemistry, molecular biology, and immunology, have played the leading role here byidentifying the headpiece, basepiece, central stalk, side stalk, cap, and in the mitochondrialenzyme, the collar around the central stalk. We can be reasonably confident also that we havea fairly good grasp of much of the high-resolution structural features of both the F1 moietycomprised of fives subunit types (, , , , and ) and parts of the F0 moiety comprised ofeither three (E. coli) or at least ten (mitochondria) subunit types. This information acquiredin several different laboratories, either by X-ray crystallography or NMR spectroscopy, includesdetails about the active site and subunit relationships. Moreover, it is consistent with recentlyreported data that the F1 moiety may be an ATP driven motor, which, during ATP synthesis,is driven in reverse by the electrochemical proton gradient generated by the electron transportchain. The real structural challenges of the future are to acquire at high resolution completeATP synthase complexes representative of different stages of the catalytic cycle during ATPsynthesis and representative also of key regulatory states. 相似文献
927.
Vik SB 《Journal of bioenergetics and biomembranes》2000,32(5):485-491
The ATP synthase from Escherichia coli is a prototype of the ATP synthases that are found in many bacteria, in the mitochondria of eukaryotes, and in the chloroplasts of plants. It contains eight different types of subunits that have traditionally been divided into F1, a water-soluble catalytic sector, and Fo, a membrane-bound ion transporting sector. In the current rotary model for ATP synthesis, the subunits can be divided into rotor and stator subunits. Several lines of evidence indicate that is one of the three rotor subunits, which rotate through 360 degrees. The three-dimensional structure of is known and its interactions with other subunits have been explored by several approaches. In light of recent work by our group and that of others, the role of in the ATP synthase from E. coli is discussed. 相似文献
928.
Methods for the preparation of an Escherichia coli tRNA mixture lacking one or a few specific tRNA species can be the basis for future applications of cell-free protein synthesis. We demonstrate here that virtually a single tRNA species in a crude E. coli tRNA mixture can be knocked out by an antisense (complementary) oligodeoxyribonucleotide. One out of five oligomers complementary to tRNAAsp blocked the aspartylation almost completely, while minimally affecting the aminoacylation with other 13 amino acids tested. This `knockout' tRNA behaved similarly to the untreated tRNA in a cell-free translation of an mRNA lacking Asp codons. 相似文献
929.
Ilia Levitin Marina Tsikalova Natalia Sazikova Vladimir Bakhmutov Zoya Starikova Aleksandr Yanovsky Yuri Struchkov 《Inorganica chimica acta》1998,270(1-2):169-176
Organocobalt(III) complexes with Schiff bases derived from a β-diketone bearing both an alkyl and an aryl group have been prepared. The template syntheses using benzoylacetone and ethylenediamine as complexing agents provide a route to alkylcobalt chelates with the corresponding tri- and tetradentate Schiff bases. However, if a β-diketone with two aryl groups, e.g. dibenzoylmethane, was employed as the starting ketoenol component, no organometallic products were detected; a new mixed-ligand ‘inorganic’ chelate of cobalt(II), [Co{O=C(Ph)CH=C(Ph)O}2(en)], was isolated instead. Its structure as well as that of one of the alkylcobalt complexes with a tridentate Schiff base composed of benzoylacetone and ethylenediamine have been established by X-ray techniques. The current scope of the template synthesis of alkylcobalt complexes with Schiff bases is summarized. 相似文献
930.
Active carbon (A.C.), after treatment at 1123 K with hydrogen for more than 24 h, was found to be very effective as a support for ruthenium catalysis in ammonia synthesis. The activity of barium promoted Ru catalysts supported on this treated A.C. is affected remarkably by the Ru precursor. Ru3(CO)12 was found previously to be the most effective Ru precursor for oxide supports such as Al2O3, MgO, and CeO2 in ammonia synthesis. However, in this study, the Ru---Bao/A.C. catalyst prepared from Ru(acac)3 was found to yield the highest activity, while the catalyst prepared from Ru3(CO)12 resulted in the lowest activity among several precursors. Transmission electron microscopy and hydrogen chemisorption showed that the particle size of Ru obtained from the decomposition of Ru(acac)3 on hydrogen-treated A.C. is smaller than the particle size of Ru obtained from the decomposition of Ru3(CO)12. Additionally, RuCl3 was found to be an effective precursor for Ru/A.C. catalyst. It has been suggested that chlorine ions can be removed easily from A.C. by hydrogen reduction at 773 K, and that this results in the high activity. 相似文献